1. Field of the Invention
The present invention relates to a magnetooptical recording medium for recording and/or reproducing information with a laser beam, utilizing the magnetooptical effect, and a method for reproducing information from the magnetooptical recording medium, and more particularly to a magnetooptical recording medium enabling high-density recording and a method for reproducing information using the medium.
2. Related Background Art
As rewritable high-density recording media, attention is being given to magnetooptical recording medium in which information is recorded by writing magnetic domains in a magnetic thin film using the thermal energy of a semiconductor laser and from which the information is read using the magnetooptical effect. In addition, there is the growing desire to further enhance the recording density of magnetooptical recording media to obtain higher-capacity recording media.
The linear recording density of optical discs including magnetooptical recording media depends largely on the laser wavelength .lambda. of the reproduction optical system used and the numerical aperture NA of an objective lens that is used. The size (spot size) of the beam of a reproduction laser light is determined once the reproducing light wavelength .lambda. and the numerical aperture NA of an objective lens are determined. The shortest mark length is about .lambda./2NA, which is the reproducible limit. Meanwhile, the track density is restricted mainly by crosstalk between adjacent tracks, and depends upon the spot size of the reproducing beam, as the shortest mark length does. Accordingly, in order to realize high recording density with the conventional optical discs, it becomes necessary to shorten the laser wavelength of the reproduction optical system or to increase the numerical aperture NA of the objective lens.
However, shortening the wavelength is not easy because it reduces efficiency, produces heat and shortens the life of the laser device. On the other hand, increasing the numerical aperture of the objective lens makes machining of the lens difficult and the distance between the lens and the optical disc becomes too short, thus raising the problem of collision between the objective lens and the optical discs.
The inventor proposed in Japanese Laid-Open Patent Application No. 6-124500 a super-resolution technique necessitating no external magnetic field upon reproduction and realizing a recording density of over the optical resolution of reproducing light (corresponding to the above spot size), and a magnetooptical recording medium suitable for the super-resolution technique.
FIG. 1A is a cross section of an example of the magnetooptical recording medium to which the super-resolution technique is applicable. The magnetooptical recording medium (optical disc) is constructed in a lamination structure of an interference layer 43, a reproducing layer 41, a memory layer 42, and a protective layer 44 formed in that order on a transparent substrate 50, and arrows shown in the reproducing layer 41 and memory layer 42 represent the directions of the iron group element sublattice magnetization in the magnetic layers. The memory layer 42 is comprised of a film with a large vertical magnetic anisotropy, for example such as TbFeCo or DyFeCo, and the reproducing layer 41 is comprised of a film which is a longitudinal magnetic layer at room temperature but turns into a vertical magnetic layer with an increase of temperature to above a threshold temperature T.sub.th. Recording information for this medium is retained by orienting the directions of magnetic domains formed in the memory layer 42 upward or downward with to the film surface.
When a light beam 38 for reproduction of information is projected on the medium of this structure from the side of substrate 50 while rotating the medium, the temperature gradient becomes as shown in FIG. 1C at the center of a data track (FIG. 1B) composed of grooves 36a, 36b and a land 37. Observing it from the side of substrate 50, there is an isotherm of temperature T.sub.th as represented by the U-shaped thick solid line in FIG. 1B, within a light spot 32 formed by the light beam 38. As described above, the reproducing layer 41 is a longitudinal magnetic layer in the region below the temperature T.sub.th (the right-upwardly hatched region in the drawing), which does not contribute to the polar Kerr effect (or which forms a front mask region 34). Thus, the recording magnetic domains retained in the memory layer 42 are masked to become invisible from the viewpoint of the magnetooptical effect. On the other hand, the reproducing layer 41 becomes a vertical magnetic layer in the region above T.sub.th (the left-upwardly hatched region in the drawing), and directions of sublattice magnetization in the reproducing layer 41 become aligned with those of the recording information in the memory layer 42 because of an exchange coupling force. As a result, the recording magnetic domains in the memory layer 42 are transferred to the reproducing layer 41 only in an aperture region 33 smaller in size than the spot 32, whereby reproducing signals can be detected only from a region smaller than the radius of light spot 32, whereby the size of recording mark 31 can be made fully smaller than the size of light spot 32, thus realizing super resolution.
Since in this super-resolution reproducing method the low-temperature region in the light spot 32, i.e., the front mask region 34 extends toward adjacent tracks, this method can increase the track density as well as the linear recording density.
In the super-resolution reproducing method as disclosed in Japanese Laid-Open Patent Application No. 6-124500, there, however, exists a magnetic domain wall between the reproducing layer and the memory layer in the magnetooptical recording medium of the two-layer structure of magnetic layers, and the magnetic domain wall permeates the reproducing layer with weaker magnetic anisotropy. Therefore, the transition of the reproducing layer from a longitudinal magnetic layer to a vertical magnetic layer with an increase in temperature occurs not steeply but gradually, which makes unclear the border between the mask region and the aperture region. If the longitudinal magnetic anisotropy of the reproducing layer is enhanced at room temperature in order to solve this problem, there occurs a problem that it becomes difficult to turn the reproducing layer into a perfect, vertical magnetic layer at the reproducing temperature. For example, if the reproducing layer is made of a heavy rare earth-iron group transition metal alloy in an RE rich state (where the magnitude of a magnetization vector of a rare earth element is greater than that of an iron group element), the rate of addition of Co is increased in order to prevent the Curie temperature of the reproducing layer from dropping and the rate of addition of the rare earth element is increased to increase the saturation magnetization M.sub.s at room temperature so as to enhance the longitudinal magnetic anisotropy, which also increases the compensation temperature at the same time, resulting in failing to decrease the saturation magnetization M.sub.s sufficiently upon reproduction and thus failing to obtain a perfect, vertical magnetic layer. In contrast, if the longitudinal anisotropy is decreased at room temperature, the perfect, vertical magnetic film can be attained at the reproducing temperature, but the border between the mask region and the aperture region becomes unclear at temperatures below it because the longitudinal anisotropy of the reproducing layer is weak. In addition, it becomes difficult for the reproducing layer to perfectly mask the magnetic information in the memory layer. Accordingly, the method in the Japanese Laid-Open Patent Application No. 6-124500 is susceptible to improvement in order to obtain good reproduction signals where the recording mark length or the track width is decreased in the medium.
Thus, the present inventor proposed a super-resolution magnetooptical recording medium (in Japanese Patent Application No. 6-45594 filed Mar. 16, 1994) in which the longitudinal anisotropy of the reproducing layer is enhanced at room temperature by providing, between the reproducing layer and the memory layer, an intermediate layer having a stronger longitudinal anisotropy at room temperature than the reproducing layer and a lower Curie temperature than the reproducing layer and in which the reproducing layer turns into a sufficient, vertical magnetic layer upon reproduction, whereby the magnetic information in the memory layer can be transferred to the reproducing layer. In this case, for example, the saturation magnetization M.sub.s of the intermediate layer is made greater than that of the reproducing layer, whereby the longitudinal anisotropy of the intermediate layer is enhanced at room temperature. Therefore, the intermediate layer is greatly influenced by the external magnetic field and the magnetostatic field from the medium at room temperature and there is a possibility that the quality of signals is degraded because the intermediate layer is influenced upon reproduction by a magnetic field generated by a magnetooptical recording apparatus (for example, a magnetic field from an external magnet for recording).